Pass Labs XA60.5 monoblock power amplifier Page 2

I began playing guitar in my teens, but switched to bass guitar when asked to join a band that already had two guitarists. I've played bass ever since, and had similar experiences with other kinds of music and ensembles I've joined over the years. With an early-music ensemble, I envisaged myself playing krummhorns and treble viols; instead I found myself playing the viola da gamba and the rackett or Wurstfagott (sausage bassoon), a double-reed ancestor of the bassoon. Playing the recorder with a baroque ensemble, I almost always played the big bass instruments. When I asked my then-recorder teacher, Nancy Winkelmann, if I could play the high-pitched treble and descant recorders, she replied that anyone could play melodies, but it was the rare musician who fully grasped the importance of the bass lines to/in ensemble playing. (I think it was a compliment.)

I dip into this bit of autobiography to explain why my sonic judgments begin with an evaluation of a component's performance in the bass. If the low frequencies don't reach a certain standard, then the shortfall can't be compensated for by good or even great sound at higher frequencies. Perhaps paradoxically, this is why I so often have preferred small speakers. While their low-frequency extension is nonexistent, their reproduction of the upper bass can have a clarity and a purityan ability to play tunes, if you willthat is too often denied larger, ported designs.

So let me get the single shortfall in the XA60.5's sound out of the way: While its low frequencies were extended and weighty, the Pass Labs amplifier's bass was also somewhat softened compared with that of the Classé CT-M600 monoblocks that have been my reference since I reviewed them in March 2011.

Back in 2005, Erick Lichte asked me to add a bass-guitar part to a studio recording the Minnesotan male-voice choir Cantus had made of the evergreen carol "Deck the Halls," which was to be included on a CD of Christmas music. I had used the graphic EQ and compression offered by a Phil Jones Bass Briefcase to modify the sound of my Fender Precision, but had then, in the final mix, added a slight boost between 50 and 100Hz to fatten the sound a bit more. The result was just what Erick needed to release Comfort and Joy: Volume Two (Cantus CTS-1205), and with the Classé monoblocks, the balance between the Fender's weight and definition sounded just as I had intended. But listening to just the drums and bass on this track with the XA60.5s, it sounded as if I'd overdone the midbass equalization by perhaps half a dB.

But this relatively minor departure from perfection didn't prevent me from appreciating what the Pass Labs did elsewhere in the audioband. I recently bought and downloaded Benjamin Zander's performance of Mahler's Symphony 2, "Resurrection," with the Philharmonia Orchestra (24-bit/192kHz ALAC files, Linn CKD 452). This was recorded by the team responsible for some of Telarc's great-sounding orchestral recordings, including Elaine Martone as co-producer and Robert Friedrich of Five/Four Productions. The "Resurrection" is an enormous, episodic work with huge orchestral climaxes contrasted against chamber-scaled sections in which a single solo instrument, a violin or a woodwind, takes the lead. Despite their modest power rating, the XA60.5s had no problem coping with the work's huge dynamic range. In one Maxellian moment almost 10 minutes into the third movement, In ruhig fliessender Bewegung, the immense scare chord blew the wind past my ears even with the less-sensitive Vivid speakers. However, the rumbling bass drum in this movement needed a little more control than the Passes could bring to bear.

The XA60.5s defined a wide, deep soundstage with this recording. In the second movement, Andante moderato, the strings' pizzicato rendering of the triple-time tune illuminated the acoustic of the Watford Colosseum, a hall I knew well back when it was known as Watford Town Hall. The same was true for the declamatory timpani at the start of the third movement, and the offstage brass choir at the start of the massive final movement was in the next borough. But the violin sections could be clearly heard to be drier than the woodwind and brass, the soundstage on this recording having a somewhat wide-angle perspective.

The fourth movement, Urlicht, has a lyric taken from Des Knaben Wunderhorn. Mezzo-soprano Sarah Connolly's opening line, "O Röschen rot!" (O little red rose!), raised goose bumps, so creamy and natural did her voice sound through the XA60.5s. As the solo violin and flute took turns weaving around her line, the image of the singer hung there between the speakers. To employ a still-meaningful cliché, her image was palpably real and fully fleshed out, rather than the two-dimensional stage flat so often produced by lesser amplifiers.

The XA60.5s didn't work their magic only with female voices. The unaccompanied choir that quietly enters two-thirds of the way through the "Resurrection"'s final movement were present in my listening room. The late Lowell George, in "20 Million Things to Do," from Little Feat's Hotcakes & Outtakes: 30 Years of Little Feat (ALAC files ripped from CD, Warner Archives/Rhino R2 79912), was resurrected in my room. The amplifier's high frequencies were neither exaggerated nor rolled off. When George knocks over the music stand in this track, it was as startling as it would have been in real life, yet without any spotlighting of the treble.

As I write these words, I'm listening to a live CD Stephen Mejias gave me, Hildur Gudnadóttir's Leyfdu Ljósinu (Touch TO:90), which I'd mentioned in my review of Pass Labs' XP-30 preamplifier. For the title track, Gudnadóttir constructs soundscapes comprising a looped motif based on a major-second interval that at first is underpinned by long, bowed notes on her cello, then by sung notes that first echo, then clash with the harmonies established by the cello. As each looped clone of the singer entered, it occupied its own place in the stage, unambiguously hanging there as the track develops. The voice is closely miked, but there was no sense of it being thrust forward at me, as it was with the Lamm M1.2 Reference monoblocks I reviewed in April 2012. When the cello reenters halfway though the track, growling octave-doubled harmonies as the voices recede into echoing, time-stretched space, it's a goose-bump moment. The Pass Labs amplifiers neither spotlit nor sweetened the sound of this inventive recording, instead quietly stepping out of the music's wayas, indeed, they should.

Summing Up
I don't have much to say about the Pass Labs XA60.5 other than this: It is the best-sounding amplifier I have ever used. It's not perfectI'd like my speakers' low frequencies controlled by a slightly tighter fistbut the XA60.5's magic midrange and sweet, detailed highs more than compensate, and are wholly addictive. And when you consider that, at $11,000/pair, the XA60.5 is relatively affordable, it's Class A all the way!

"Nelson Pass says that the actual chip temperatures are about 70°C (158°F) below their rated maximum. But with that much bias current, a class-A amplifier will run hot, "about 25°C [77°F] above ambient,"

Is it possible an amp so old like 33H to compare with something so new with all these new materials and design? I think that 60,5 probably outperform 33H in every way with no direct comparisons needed at all. Dont you?

Thanks so much for the great review. I have a quick question, Do you recall if the front meter was contantly moving when driving the Alexias at normal listening levels? (e.g. around 70-80db). Did you feel like the XA 60.5 had enough power for the Alexias or did you feel like a higher XA, such as the XA100.5, would be a much better match?

What are your thoughts regarding either the 30.5 or the 60.5 running Genesis 500 speakers? I'd love to gain the sonic qualities these have and with the 500's being self powered on the bass, I'd like to think it might be possible.

Hello John, I keep reading fantastic things about this amp but I am
hesitating because I do not know My speakers sensitivity or Nominal
Impedance. I have Dunlavy SC-V but no manual or any technical info.
Will this amp work with those ?
Regards,
Claude Panneton

This implies that a single XA30.5 and a pair of X60.5 amps will both put out 130W per channel), making the stereo chassis twice as powerful as a single xa60.5 monoblock chasis. You mentioned that the XA30.5 is a full 6db more powerful than its rating, but it looks like the XA60.5 is measuring only 3db more powerful than it’s 60w rating.

Your 4 ohm measurements were reported similarly.

On the 2 Ohm measurements, you actually state the XA30.5 was driving a single channel, so that result looks consistent.

What are the correct values? Were the numbers for the XA30.5 actually for a single channel?